1. Field of the Invention:
This invention relates generally to an adaptive control system for improving the performance of power producing machines such as internal combustion engines or the like, and more specifically to a closed-loop control system which remains stable over wide variations in engine speed settings.
2. Discussion of the Prior Art:
In the Schweizer et al. U.S. Pat. No. 4,026,251, there is described a closed-loop digital electronic control system for an engine in which a machine controlling parameter is perturbated (dithered) about a given setting and the performance of the machine is monitored to determine whether movement of the machine controlling parameter about the given setting improved or degraded the system's performance. Where a given movement of the setting resulted in improved performance, the resulting control signal developed by the electronic circuits is used to create a further movement of the control setting in the same direction. However, if the small change introduced results in degraded performance, then the machine setting is moved in the opposite direction.
In the device of the aforereferenced Schweitzer et al patent, engine performance is monitored by accumulating "celsig" pulses during predetermined segments of the dithering cycle. "Celsig" pulses are produced by the rotating engine shaft, flywheel or alternator and their rate of occurrence is proportional to engine speed. The control system logic utilizes a counting cycle which is divided into four quarters. Counting means are employed to count the number of "celsig" pulses in each quarter of the dither cycle. The counter is used to count up for the first quarter, down for the next two quarters and up again during the fourth quarter. The net number of counts remaining in the up/down counter at the end of each counting cycle is then used to determine whether to adjust the machine control parameter setting in one direction or the opposite direction. These corrections continue until no significant change in engine speed is produced by the dithering of the machine control parameter, indicating that the appropriate setting for maximum brake torque (MBT) has been located.
It has been found that the system works well provided that the number of "celsig" pulses is sufficiently large and that the dither period contains a large number of engine cycles, i.e., the engine rotational frequency is much larger than the dither frequency. However, if the engine frequency is not sufficiently larger than the dither frequency, loss of control may result.
It has been theorized that this loss of control is due to the fact that in the prior system, the dither pulses are asynchronous with respect to the engine cycle of the machine being controlled. That is to say, in the device of the aforereferenced Schweitzer et al. patent, the dither frequency is a fixed parameter and determined strictly by the electronic oscillator used to generate it. As such, the change-over from "advance" to "retard" or vice versa at the initiation of or midway through the dither period is independent of the rotation of the engine shaft and of the firing pulses acting upon the pistons. As a result, and as will be set forth in greater detail hereinbelow, the normal variations in engine speed between cylinder-to-cylinder firings may at some times be reinforced by the speed changes occasioned by the dithering of the machine's control parameter setting and, at other times, be in phase opposition with the speed changes produced by the dithering.